So this is all speculation but here are some of the things that I have heard/read on ESPN and ESPN.com today.
On the play just before the head butt the Italian defender twisted Zidane's nipple.
The Italian guy called Zidane(whose parents immigrated to France from Algeria) a "dirty terrorist."
Now I think it is hard to justify what Zidane did but a racial slur like this would come close in my eyes.